I'd do it every 50K or so.
It's pretty easy, drain the system by disconnecting a hose, or get a turkey baster and suck it out of the reservoir. Then fill the reservoir, start the engine, and start turning the wheel from lock-to-lock over and over to get the air out. If you use the baster method, you don't get all the old fluid out, so it couldn't hurt to do it again.